1. Field of the Invention
The present invention relates to a precursor of a superconducting wire having a core of MgB2.
2. Description of Related Art
MgB2 is a superconductor discovered in the year 2001. Since MgB2 has the highest critical temperature (of 39 K) among metallic superconductors, this superconductor may make it possible to operate superconducting machines at a temperature of 10 to 20 K, which is higher than conventional superconducting-machine-operating temperatures. Thus, the application thereof in this field has been expected. The expectation has been directed, in particular, to application to machines using a magnetic field very small in fluctuation with time, such as nuclear magnetic resonance spectrometers, and magnetic resonance imaging diagnostic machines for medicine. This is because MgB2 hardly causes a problem of flux creep which arises remarkably in cuprate superconductors or others.
MgB2 gives a practical critical current density in the case of filling a powder thereof into a metallic tube, subjecting the workpiece to area-reducing working to be made into a wire, and then firing the wire. This wire-producing method is called the powder-in-tube method. The powder-in-tube method, which may be referred to as the “PIT method” hereinafter, is roughly classified into two methods in accordance with the species of the powder to be filled.
One of the two is a method of using Mg (magnesium) powder and B (boron) powder as the powder to be filled, and firing the powders to produce MgB2. This is called the in-situ method. The other is a method of using MgB2 powder (magnesium diboride powder) as the powder to be filled, and firing the powder to strengthen bonding between particles of the MgB2 powder. This is called the ex-situ method.
Japanese Patent Laid-open No. 2007-157590 (Patent Document 1) discloses a method for producing a magnesium diboride superconducting wire by the powder-in-tube method, the method including the steps of arranging a metallic core into a powder inside a metallic tube, and then subjecting the workpiece to area-reducing working provided that a thin-film layer of components to be introduced into a superconductor to be obtained is laid onto the metallic core or metallic tube in advance before the working, whereby strong power in the area-reducing working step is used to disperse the components into the superconductor.
G. Ginuchi et al. “Grain size effects on superconducting properties of high density bulk MgB2” Supercond. Sci. Technol. 17 (9) (2004) 5583-588 (Non-Patent Document 1) states that it is desired that B powder as the raw material is a fine amorphous powder.
Katsura et al. “Particle Size Reinforcing Factors and Grain Boundary Pinning of In-situ Method MgB2 Bulk” Cryogenic Engineering, Vol. 41 (2006) No. 11 pp. 497-504 (Non-Patent Document 2) describes a matter that particle-size-reinforcing factors of a non-doped in-situ MgB2 bulk are the particle size of raw-material boron (B), the thermal treatment temperature, the ratio between components of the starting material, and others; a matter that the production of MgB2 high in grain boundary density is effective for improving the critical current density Jc property of the MgB2 bulk in a high magnetic field; and others.